The epicentre was in the Ionian Sea, about 130km southwest of Patras in the Peloponnese, the USGS said.
The tremor was felt in Greece's capital, Athens, and as far away as Italy, Malta, Albania and Libya.
People walk past damages in a street of Zante after the quake.
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Was quake felt on Zante, Crete and Corfu? .
In Zakynthos, the harbour is still functional although local schools have been shut down for the day as a precaution.
The natural disaster was felt across southern Italy, including in the regions of Sicily, Basilicata, Calabria, Puglia and Campania. However, power outages have been reported on the island of Zakynthos, according to BNO. Earthquake-resistant buildings on Greek island Zakynthos survive 6.4 quake: "no injuries reported", said the United Nations office for disaster and risk reduction.
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Three people were taken to hospital, a spokesman for Greece's civil protection agency told Reuters.
'The energy unleashed, based on the angle of the faultline, fanned out towards Italy, ' he added. Firefighters in Calabria, Puglia and Sicily received thousands of telephone calls from anxious locals. Subsequently, it reported an "anomaly of some 10 centimeters observed by the marigraph station in Le Castella, in the province of Crotone, confirming an orange tsunami alert for Italy's Ionian coast".
The quake rattled the whole of western Greece and was felt in Kefalonia, Corfu, and Crete according to the USGS. You should familiarise yourself with safety procedures in the event of an natural disaster and follow advice given by the local authorities.
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"The natural disaster happened exactly where the two lithospheric plates of Africa and Europe meet", said Efthimios Lekkas, professor of geology at the University of Athens.